1. Field of the Invention
The present invention relates to a heat generating element with at least one PTC element, contact sheets flatly lying against it on either side, a housing forming at least one opening for receiving the at least one PTC element and having a terminal side where contact studs allocated to the contact sheets are exposed. The heat generating element according to the invention includes a wedge element with a broader and a narrower end face which are connected to each other via first and second wedge surfaces, the first wedge surface extending in parallel to one of the contact sheets and the second wedge surface being exposed at the outer side of the housing.
2. Description of the Related Art
Such a heat generating element is known from EP 1 921 896 A1. In this prior art, the housing is formed by two housing elements which can be connected to each other and are each formed as molded plastic parts. The contact sheet and insulation layers respectively provided at the outer side of it are coated with the plastic material forming the respective housing element to form a unit of contact sheet, insulation layer and housing element.
Such heat generating elements are in particular employed for heating liquids, for example in a motor vehicle. An electric heating device that normally accommodates several heat generating elements has one or several pockets extending in parallel with respect to each other. The pockets separate a circulation chamber for the fluid to be heated from a terminal side in which normally the electric contacts are exposed and connected. Since the chambers are formed from a metallic and thus electrically conductive material, for the operational reliability of the electric heating device, good electric insulation between the heat generating element inserted in the pocket and the walls delimiting the pocket inside is very important.
The generic heat generating elements are usually first inserted into the pocket. Then, the wedge element is shifted relative to the internal wall of the pocket and an outer surface of the heat generating element to wedge up the heat generating element in the pocket. The wedge element is lying with its first wedge surface in parallel to one of the contact sheets, usually with an insulation layer being inserted, and with its second wedge surface usually in parallel to a slightly beveled inner surface of the pocket. Correspondingly, through the medium of the wedge element, a flat contact between the heat generating element and the pocket results.
On one side, the other contact sheet lies against the inner surface of the pocket, usually also with an insulation layer being inserted, while on the other side, the wedge element is inserted. This arrangement is essential for an effective employment of the heat generating element as due to the self-regulating properties of the PTC elements, good, preferably symmetric and thus two-sided heat dissipation must be taken care of. Otherwise, the PTC element will get too hot and the self-regulating properties of the PTC element will prevent further absorption of electric energy for heat generation.
With the generic heat generating element, various problems arise. For example, when the wedge element is shifted, there is a risk in that the layers of a layer structure within the heat generating element are shifted with respect to each other. These layers include—from inside to outside—the PTC element, the contact sheets lying against it on either side, the insulation layers usually provided at their outer sides, where it is also possible to allocate several different insulation layers to each individual contact sheet, and finally the wedge element itself. In the above-mentioned prior art, the contact sheet is only inserted into the housing. At the base, it is supported on an edge of the housing. Furthermore, there is a problem in that on the one hand, the contact sheet must be all-over surrounded by an insulation layer which insulation layer must also cover edge regions of the contact sheet to reliably prevent an exposure of electrically conductive surfaces at the outer side of the heat generating element. The coating of the contact sheet together with the insulation layer known from prior art can lead to defects, in particular at the insulation layer, due to heat introduction during the injection molding process and/or due to the injection pressures prevailing in the process. Already microscopic fissures can cause high rejections as these are not correctly identified during production, so that defective heat generating elements are installed into the electric heating device and make it unusable altogether.